When attempting to reduce the emissions of nitrogen oxides (NOx) from internal combustion engines, many efforts have been made to modify the combustion conditions in order to reduce the NOx-emissions, while still maintaining the combustion efficiency at a satisfactory level.
Among the traditional techniques used for the reduction of NOx-emissions, inter alia, the technique of Exhaust Gas Recirculation (EGR) may be mentioned, as well as special designs of fuel injectors and combustion chambers. Other important parameters are compression, fuel injection time and fuel injection pressure. Techniques involving water injection, the use of fuel/water emulsions, and so-called Selective Catalytic Reduction (SCR) by ammonia, have also been employed. It has thus been found that a one-sided optimization of the combustion efficiency often results in increased NOx-emissions.
It is presently required that both the fuel consumption and the NOx-emissions be reduced. There are also strong demands on reduced emissions of other chemical compounds which are potentially hazardous to the environment, e.g. hydrocarbons.
Accordingly, there is an increased need for catalytic converters which are also able to treat exhaust gases from so-called Lean Combustion (LC) engines. Therefore, a number of different catalytic converters have been developed and are well-known from commercial applications in e.g. motor vehicles.
Typically, conventional catalytic converters comprise one or several matrices, or monolith bricks as they are sometimes called. Such bricks or monoliths are in the form of a ceramic honeycomb substrate, with through passages or cells, and which can be furnished with a porous surface coating. Particles of a suitable catalyst are embedded in the surface of the matrix, and the design of the matrix has been optimized in order to maximise the surface area over which catalytic reactions take place. Common catalysts are noble metals, e.g. silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), gallium (Ga) or ruthenium (Ru) or mixtures thereof. There are also a number of other metals and metal oxides which may be used as catalysts. Such catalysts may have the ability to catalyse oxidation or reduction reactions, or both.
It is also previously known to use crystalline aluminium silicates, so-called zeolites, loaded with a suitable catalyst. The use of zeolites in connection with the catalytic conversion of exhaust gases is disclosed, e.g in European Application Nos. 499,931 A1 and 445,408 A.
Furthermore, it is also known to combine several different catalytic matrices, or to arrange a so-called after-burner in the catalytic conversion process. Such arrangements are disclosed, e.g. in U.S. Pat. No. 5,465,574.
It is also known to use a honeycomb monolith of corrugated metal foil, having a suitable catalyst carried or supported on its surface.
It has also been suggested, e.g. in European Application No. 483,708 A1, to combine a conventional ceramic catalytic converter with an electrically heatable catalytic converter, in order to ensure that the optimum temperature for catalytic conversion is maintained.
Thus, a number of different catalyst materials, devices, and arrangements for the catalytic conversion of exhaust gases have been described in the art.
It is therefore believed that simultaneous elimination of nitrogen oxides (NOx) and hydrocarbons (HxCy) may take place over e.g. an Ag-catalyst, according to the following (simplified) chemical reactions:
A)NOx + HxCy → N2 + H2O + CO2 + COandB)O2 + HxCy → H2O + CO2
However, in practice, it has been found that the following reaction is predominant:
C)NO2 + HxCy → N2 + H2O + CO2
It should be noted that the term HxCy in these chemical reactions not only refers to hydrocarbons but is also relevant for other reducing agents which further comprise oxygen and/or sulphur. Accordingly, the reducing agent HxCy could also be expressed as HxCyOzSw. Examples of reducing agents which might be present in exhaust gases are alkanes, alkenes, paraffins, alcohols, aldehydes, ketones, ethers or esters, and different sulphur-containing compounds. Also CO or H2 could act as reducing agents. The reducing agent in the exhaust gases can originate from the fuel or the combustion air, or it can be added to the exhaust gases on purpose.
It has been found that the above-mentioned reaction according to C) is very rapid over e.g. Ag-catalysts. Acidic catalysts (H+) and acidic zeolites, doped with Ag or other suitable catalysts, have been found to be selective in the sense that NO2 will readily be converted, whereas NO will not. This can be a great disadvantage, since NO is predominant in “lean” exhaust gases from e.g. LC-engines. Another problem is that the available amount of NO2 can become limiting for the reduction of hydrocarbons (HxCy) or other undesired compounds.
In order to solve this problem, i.e. to be able to reduce the amount of both NO and HxCy in the exhaust gases, it has been suggested to combine an Ag-zeolite catalyst with a Pt-catalyst. Normally, the following main reactions will take place over a conventional Pt-catalyst:
D)NO + 1/2 O2 ⇄ NO2E)O2 + HxCy → H2O + CO2F)2 NO + HxCy → N2O + H2O + CO2
When using a conventional Ag-zeolite catalyst in combination with a conventional Pt-catalyst, all four reactions C), D), E) and F) will occur. However, since hydrocarbons (HxCy) are consumed in the chemical reactions E) and F), there is a risk that there will not be a sufficient amount of hydrocarbon (HxCy) left for the reaction with nitrogen dioxide (NO2), according to reaction C). This results in an undesired residue of nitrogen dioxide (NO2) in the catalytically converted exhaust gases, originating from reaction D).
Previous attempts have been made to solve this problem with different types of catalysts, by means of combining different catalysts, and by means of adding an additional amount of hydrocarbon to the exhaust gases in order to supply the reaction C) with a sufficient amount of hydrocarbon.
However, many of the previous solutions have been associated with the problem of undesired oxidation of hydrocarbons (HxCy) over at least some surfaces of the oxidation catalyst, which preferably should only catalyse oxidation of nitrogen monoxide (NO) into nitrogen dioxide (NO2), according to reaction D).
Another problem associated with many previously known catalysts is that, during certain conditions, they will catalyse reaction F), which produces dinitrogen oxide (N2O). This reaction is undesired, and it is preferred that the nitrogen oxides (NOx) in the exhaust gases are converted into nitrogen (N2) to the highest possible degree, and not into dinitrogen oxide (N2O).
Accordingly, there is a need for a new, selective oxidation catalyst material, which catalyses oxidation of nitrogen monoxide (NO) into nitrogen dioxide (NO2) and which does not catalyse oxidation of hydrocarbons.
Furthermore, there is also a need for an effective combination of such a selective oxidation catalyst material, catalysing a reaction which produces nitrogen dioxide (NO2), and a reduction catalyst material, catalysing a reaction in which nitrogen dioxide (NO2) is reduced by hydrocarbons or other reducing agents into nitrogen (N2).
Accordingly, one object of the present invention is to provide a porous material for catalytic conversion of exhaust gases, by means of which porous material it is possible to selectively catalyse the oxidation of nitrogen monoxide (NO) into nitrogen dioxide (NO2), and avoid catalytic oxidation of hydrocarbons (HxCy) or other reducing agents.
A second object of the present invention is to provide a porous material for catalytic conversion of exhaust gases, wherein primarily only the desired reactions take place, as is a result of which the contents of NO, NO2 and HxCy in the catalytically converted exhaust gases are effectively decreased, and the resulting conversion products primarily are N2, CO2 and H2O, and not N2O.
A third object of the present invention is to provide a method for the catalytic conversion of exhaust gases in which such a porous material is utilized.
A fourth object of the present invention is to provide an advantageous use of porous materials therefore.
A fifth object of the present invention is to provide an advantageous arrangement for the catalytic conversion of exhaust gases utilizing such a porous material.